Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having a low spin rate on driver shots. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator and (d) a carboxylic acid having a benzene ring and/or a salt thereof, provided that the rubber composition further contains (e) a metal compound in case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

CROSS REFERENCE

The present application is a 37 C.F.R. §1.53(b) continuation of, andclaims priority to, U.S. application Ser. No. 13/728,810, filed Dec. 27,2012. Priority is also claimed to Japanese Application No. 2012-166101filed on Jul. 26, 2012 and Japanese Application No. 2011-289985 on Dec.28, 2011. The entire contents of each of these applications are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a golf ball having a low spin rate ondriver shots, in particular, to an improvement of a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for improving a flight distance on driver shots, forexample, there are methods of using a core having high resilience andusing a core having a hardness distribution in which the hardnessincreases toward the surface of the core from the center thereof. Theformer method has an effect of enhancing an initial speed, and thelatter method has an effect of a higher launch angle and a lower spinrate. A golf ball having a higher launch angle and a low spin ratetravels a great distance.

For example, Japanese Patent Publications Nos. S61-37178 A, S61-113475A, S61-253079 A, 2008-212681 A, 2008-523952 T and 2009-119256 A disclosea technique of enhancing resilience of the core. Japanese PatentPublications Nos. S61-37178 A and S61-113475 A disclose a solid golfball having an inner core where zinc acrylate as a co-crosslinkingagent, palmitic acid, stearic acid, or myristic acid as aco-crosslinking activator, zinc oxide as another co-crosslinkingactivator, and a reaction rate retarder are blended, with respect to 100parts by weight of a rubber.

Japanese Patent Publication No. S61-253079 A discloses a solid golf ballformed from a rubber composition containing an α,β-unsaturatedcarboxylic acid in an amount of 15 parts to 35 parts by weight, a metalcompound to react with the α,β-unsaturated carboxylic acid and form asalt thereof in an amount of 7 parts to 60 parts by weight, and a highfatty acid metal salt in an amount of 1 part to 10 parts by weight withrespect to 100 parts by weight of a base rubber.

Japanese Patent Publication No. 2008-212681 A discloses a golf ballcomprising, as a component, a molded and crosslinked product obtainedfrom a rubber composition essentially comprising a base rubber, afiller, an organic peroxide, an α,β-unsaturated carboxylic acid and/or ametal salt thereof, a copper salt of a saturated or unsaturated fattyacid.

Japanese Patent Publication No. 2008-523952 T discloses a golf ball, ora component thereof, molded from a composition comprising a baseelastomer selected from the group consisting of polybutadiene andmixtures of polybutadiene with other elastomers, at least one metallicsalt of an unsaturated monocarboxylic acid, a free radical initiator,and a non-conjugated diene monomer.

Japanese Patent Publication No. 2009-119256 A discloses a method ofmanufacturing a golf ball, comprising preparing a masterbatch of anunsaturated carboxylic acid and/or a metal salt thereof by mixing theunsaturated carboxylic acid and/or the metal salt thereof with a rubbermaterial ahead, using the masterbatch to prepare a rubber compositioncontaining the rubber material, and employing a heated and moldedproduct of the rubber composition as a golf ball component, wherein themasterbatch of the unsaturated carboxylic acid and/or the metal saltthereof comprises; (A) from 20 wt % to 100 wt % of a modifiedpolybutadiene obtained by modifying a polybutadiene having a vinylcontent of from 0 to 2%, a cis-1,4 bond content of at least 80% andactive terminals, the active terminal being modified with at least onetype of alkoxysilane compound, and (B) from 80 wt % to 0 wt % of a dienerubber other than (A) the above rubber component [the figures arerepresented by wt % in the case that a total amount of (A) and (B) equalto 100 wt %] and (C) an unsaturated carboxylic acid and/or a metal saltthereof.

For example, Japanese Patent Publications Nos. H6-154357 A, 2008-194471A, 2008-194473 A and 2010-253268 A disclose a core having a hardnessdistribution. Japanese Patent Publication No. H6-154357 A discloses atwo-piece golf ball comprising a core formed of a rubber compositioncontaining a base rubber, a co-crosslinking agent, and an organicperoxide, and a cover covering said core, wherein the core has thefollowing hardness distribution according to JIS-C type hardness meterreadings: (1) hardness at center: 58-73, (2) hardness at 5 to 10 mm fromcenter: 65-75, (3) hardness at 15 mm from center: 74-82, (4) surfacehardness: 76-84, wherein hardness (2) is almost constant within theabove range, and the relation (1)<(2)<(3)≦(4) is satisfied.

Japanese Patent Publication No. 2008-194471 A discloses a solid golfball comprising a solid core and a cover layer that encases the core,wherein the solid core is formed of a rubber composition composed of 100parts by weight of a base rubber that includes from 60 to 100 parts byweight of a polybutadiene rubber having a cis-1,4 bond content of atleast 60% and synthesized using a rare-earth catalyst, from 0.1 to 5parts by weight of an organic sulfur compound, an unsaturated carboxylicacid or a metal salt thereof, an inorganic filler, and an antioxidant;the solid core has a deformation from 2.0 mm to 4.0 mm, when applying aload from an initial load of 10 kgf to a final load of 130 kgf and hasthe hardness distribution shown in the following table.

TABLE 1 Hardness distribution in solid core Shore D harness Center 30 to48 Region located 4 mm from center 34 to 52 Region located 8 mm fromcenter 40 to 58 Region located 12 mm from center (Q) 43 to 61 Regionlocated 2 to 3 mm inside of surface (R) 36 to 54 Surface (S) 41 to 59Hardness difference [(Q) − (S)]  1 to 10 Hardness difference [(S) − (R)] 3 to 10

Japanese Patent Publication No. 2008-194473 A discloses a solid golfball comprising a solid core and a cover layer that encases the core,wherein the solid core is formed of a rubber composition composed of 100parts by weight of a base rubber that includes from 60 to 100 parts byweight of a polybutadiene rubber having a cis-1,4 bond content of atleast 60% and synthesized using a rare-earth catalyst, from 0.1 to 5parts by weight of an organic sulfur compound, an unsaturated carboxylicacid or a metal salt thereof, and an inorganic filler; the solid corehas a deformation from 2.0 mm to 4.0 mm, when applying a load from aninitial load of 10 kgf to a final load of 130 kgf and has the hardnessdistribution shown in the following table.

TABLE 2 Hardness distribution in solid core Shore D harness Center 25 to45 Region located 5 to 10 mm from center 39 to 58 Region located 15 mmfrom center 36 to 55 Surface (S) 55 to 75 Hardness difference 20 to 50between center and surface

Japanese Patent Publication No. 2010-253268 A discloses a multi-piecesolid golf ball comprising a core, an envelope layer encasing the core,an intermediate layer encasing the envelope layer, and a cover whichencases the intermediate layer and has formed on a surface thereof aplurality of dimples, wherein the core is formed primarily of a rubbermaterial and has a hardness which gradually increases from a center to asurface thereof, the hardness difference in JIS-C hardness units betweenthe core center and the core surface being at least 15 and, letting (I)be the average value for cross-sectional hardness at a position about 15mm from the core center and at the core center and letting (II) be thecross-sectional hardness at a position about 7.5 mm from the corecenter, the hardness difference (I)-(II) in JIS-C units being within ±2;and the envelope layer, intermediate layer and cover have hardness whichsatisfy the condition: cover hardness>intermediate layerhardness>envelope layer hardness.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball having alow spin rate on driver shots.

The present invention provides a golf ball comprising a spherical coreand at least one cover layer covering the spherical core, wherein thespherical core is formed from a rubber composition containing (a) a baserubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof as a co-crosslinking agent, (c) acrosslinking initiator, and (d) a carboxylic acid having a benzene ringand/or a salt thereof, provided that the rubber composition furthercontains (e) a metal compound in the case of containing only (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent. The present invention is configured as describedabove so that the spherical core has a higher degree of an outer-hardinner-soft structure where a surface hardness of the spherical core ishigher than a center hardness thereof. A golf ball having a sphericalcore with a higher degree of the outer-hard inner-soft structure isexpected to reduce the spin rate on driver shots and provide a greaterflight distance.

The reason why the spherical core of the golf ball of the presentinvention has a higher degree of the outer-hard inner-soft structure isconsidered as follows. It is conceivable that (b) the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms blended inthe rubber composition forms an ion cluster in the core, resulting in ametal crosslinking of a rubber molecular chain. By blending (d) thecarboxylic acid having the benzene ring and/or the salt thereof in therubber composition, (d) the carboxylic acid and/or the salt thereofexchanges a cation with the ion cluster formed by the metal salt of (b)the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, therebybreaking the metal crosslinking by the metal salt of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. This cationexchange reaction easily occurs at the core central part where thetemperature is high, and less occurs toward the core surface. Whenmolding the core, the internal temperature of the core is high at thecore central part and decreases toward the core surface, since reactionheat from a curing reaction of the rubber composition accumulates at thecore central part. That is, the breaking of the metal crosslinking by(d) the carboxylic acid having the benzene ring and/or the salt thereofeasily occurs at the core central part, but less occurs at toward thesurface. As a result, it is conceivable that since a crosslinkingdensity in the core increases from the center of the core toward thesurface thereof, the core hardness increases from the center of the coretoward the surface thereof.

The present invention provides a golf ball having a low spin rate ondriver shots.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing the golf ballaccording to the preferable embodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the sphericalcore;

FIG. 3 is a graph showing the hardness distribution of the sphericalcore;

FIG. 4 is a graph showing the hardness distribution of the sphericalcore;

FIG. 5 is a graph showing the hardness distribution of the sphericalcore;

FIG. 6 is a graph showing the hardness distribution of the sphericalcore;

FIG. 7 is a graph showing the hardness distribution of the sphericalcore;

FIG. 8 is a graph showing the hardness distribution of the sphericalcore;

FIG. 9 is a graph showing the hardness distribution of the sphericalcore;

FIG. 10 is a graph showing the hardness distribution of the sphericalcore;

FIG. 11 is a graph showing the hardness distribution of the sphericalcore;

FIG. 12 is a graph showing the hardness distribution of the sphericalcore;

FIG. 13 is a graph showing the hardness distribution of the sphericalcore;

FIG. 14 is a graph showing the hardness distribution of the sphericalcore;

FIG. 15 is a graph showing the hardness distribution of the sphericalcore;

FIG. 16 is a graph showing the hardness distribution of the sphericalcore;

FIG. 17 is a graph showing the hardness distribution of the sphericalcore;

FIG. 18 is a graph showing the hardness distribution of the sphericalcore;

FIG. 19 is a graph showing the hardness distribution of the sphericalcore;

FIG. 20 is a graph showing the hardness distribution of the sphericalcore;

FIG. 21 is a graph showing the hardness distribution of the sphericalcore;

FIG. 22 is a graph showing the hardness distribution of the sphericalcore;

FIG. 23 is a graph showing the hardness distribution of the sphericalcore;

FIG. 24 is a graph showing the hardness distribution of the sphericalcore;

FIG. 25 is a graph showing the hardness distribution of the sphericalcore;

FIG. 26 is a graph showing the hardness distribution of the sphericalcore;

FIG. 27 is a graph showing the hardness distribution of the sphericalcore;

FIG. 28 is a graph showing the hardness distribution of the sphericalcore;

FIG. 29 is a graph showing the hardness distribution of the sphericalcore;

FIG. 30 is a graph showing the hardness distribution of the sphericalcore; and

FIG. 31 is a graph showing the hardness distribution of the sphericalcore.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The golf ball of the present invention has a spherical core and at leastone cover layer covering the spherical core, wherein the spherical coreis formed from a rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator and (d) a carboxylic acid having a benzene ring and/or a saltthereof, provided that the rubber composition further contains (e) ametal compound in the case of containing only (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

First, (a) the base rubber used in the present invention will beexplained. As (a) the base rubber used in the present invention, naturalrubber and/or synthetic rubber can be used. For example, polybutadienerubber, natural rubber, polyisoprene rubber, styrene polybutadienerubber, ethylene-propylene-diene rubber (EPDM), or the like can be used.These rubbers may be used solely or two or more of these rubbers may beused in combination. Among them, typically preferred is the highcis-polybutadiene having a cis-1,4 bond in a proportion of 40% or more,more preferably 80% or more, even more preferably 90% or more in view ofits superior resilience property.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in a contentof 2 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. If the content of 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cis polybutadiene preferably includes one synthesized using arare earth element catalyst. When a neodymium catalyst, which employs aneodymium compound of a lanthanum series rare earth element compound, isused, a polybutadiene rubber having a high content of a cis-1,4 bond anda low content of a 1,2-vinyl bond is obtained with excellentpolymerization activity. Such a polybutadiene rubber is particularlypreferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or more, more preferably 32 or more, even morepreferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140 or less, more preferably 120 or less, even morepreferably 100 or less, and most preferably 80 or less. It is noted thatthe Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is avalue measured according to JIS K6300 using an L rotor under theconditions of: a preheating time of 1 minute; a rotor revolution time of4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cis polybutadiene is excessively low, the processability maydeteriorate. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(manufactured by Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

Next, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or a metal salt thereof will be explained. (b) The α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofis blended as a co-crosslinking agent in the rubber composition and hasan action of crosslinking a rubber molecule by graft polymerization to abase rubber molecular chain. In the case that the rubber compositionused in the present invention contains only the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent,the rubber composition preferably further contains (e) a metal compoundas an essential component. Neutralizing the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms with the metal compound in the rubbercomposition provides substantially the same effect as using the metalsalt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.Further, in the case of using the α,β-unsaturated carboxylic acid having3 to 8 carbon atoms and the metal salt thereof in combination, (e) themetal compound may be used as an optional component.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes,for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid,crotonic acid, and the like.

Examples of the metals constituting the metal salts of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include:monovalent metal ions such as sodium, potassium, lithium or the like;divalent metal ions such as magnesium, calcium, zinc, barium, cadmium orthe like; trivalent metal ions such as aluminum or the like; and othermetal ions such as tin, zirconium or the like. The above metal ions canbe used solely or as a mixture of at least two of them. Among thesemetal ions, divalent metal ions such as magnesium, calcium, zinc,barium, cadmium or the like are preferable. Use of the divalent metalsalts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomseasily generates a metal crosslinking between the rubber molecules.Especially, as the divalent metal salt, zinc acrylate is preferable,because the zinc acrylate enhances the resilience of the resultant golfball. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or a metal salt thereof may be used solely or in combination of atleast two of them.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and ispreferably 50 parts by mass or less, more preferably 45 parts by mass orless, even more preferably 35 parts by mass or less, with respect to 100parts by mass of (a) the base rubber. If the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof is less than 15 parts by mass, the content of (c) theco-crosslinking initiator which will be explained below must beincreased in order to obtain the appropriate hardness of theconstituting member formed from the rubber composition, which tends tocause the lower resilience. On the other hand, if the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof exceeds 50 parts by mass, the constituting memberformed from the rubber composition becomes excessively hard, which tendsto cause the lower shot feeling.

(c) The crosslinking initiator is blended in order to crosslink (a) thebase rubber component. As (c) the crosslinking initiator, an organicperoxide is preferred. Specific examples of the organic peroxide includeorganic peroxides such as dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Theseorganic peroxides may be used solely or two or more of these organicperoxides may be used in combination. Among them, dicumyl peroxide ispreferably used.

The content of (c) the crosslinking initiator is preferably 0.2 part bymass or more, and more preferably 0.5 part by mass or more, and ispreferably 5.0 parts by mass or less, and more preferably 2.5 parts bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the content of (c) the crosslinking initiator is less than 0.2 partby mass, the constituting member formed from the rubber compositionbecomes too soft, and thus the golf ball may have the lower resilience.If the content of (c) the crosslinking initiator exceeds 5.0 parts bymass, the amount of (b) the co-crosslinking agent must be decreased inorder to obtain the appropriate hardness of the constituting memberformed from the rubber composition, resulting in the insufficientresilience and lower durability of the golf ball.

Next, (d) the carboxylic acid having the benzene ring and/or the saltthereof will be explained. (d) The carboxylic acid having the benzenering and/or the salt thereof is not limited, as long as it is a compoundhaving the benzene ring and a carboxyl group and/or a salt thereof.Examples thereof include an aromatic carboxylic acid having a carboxylgroup directly bonded to a benzene ring, an aromatic-aliphaticcarboxylic acid having an aliphatic carboxylic acid bonded to a benzenering, a polynuclear aromatic carboxylic acid having a carboxyl groupdirectly bonded to a fused benzene ring, and a polynucleararomatic-aliphatic carboxylic acid having an aliphatic carboxylic acidbonded to a fused benzene ring. As the fused benzene ring structure,naphthalene, anthracene, phenalene, phenanthrene, tetracene, pyrene andthe like are included. It is conceivable that (d) the carboxylic acidhaving the benzene ring and/or the salt thereof has an action ofbreaking the metal crosslinking by (b) the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms at the corecentral part, when molding the core.

The number of the carboxyl group of (d) the carboxylic acid having thebenzene ring may be one (monocarboxylic acid), or two or more(polycarboxylic acid), but one is preferable. The benzene ring or thefused benzene ring may have another substituent group directly bonded tothe benzene ring or the fused benzene ring other than the carboxylgroup. The substituent group includes, for example, an alkyl group(preferably, an alkyl group having 1 to 4 carbon atoms), an aryl group(preferably, a phenyl group), an amino group, a hydroxyl group, analkoxyl group (preferably, an alkoxyl group having 1 to 4 carbon atoms),an oxo group, a halogen group, and the like.

Specific examples of the aromatic carboxylic acid having the carboxylgroup directly bonded to the benzene ring include, for example, benzoicacid (C7), phthalic acid (C8), isophthalic acid (C8), terephthalic acid(C8), hemimellitic acid (benzene-1,2,3-tricarboxylic acid) (C9),trimellitic acid (benzene-1,2,4-tricarboxylic acid) (C9), trimesic acid(benzene-1,3,5-tricarboxylic acid) (C9), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid) (C10), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid) (C10), pyromellitic acid(benzene-1,2,4,5-tetracarboxylic acid) (C10), mellitic acid (benzenehexacarboxylic acid) (C12), and the like. Specific examples of thearomatic-aliphatic carboxylic acid having the aliphatic carboxylic acidbonded to the benzene ring include α-toluic acid (phenylacetic acid)(C8), hydratropic acid (2-phenylpropanoic acid), hydrocinnamic acid(3-phenylpropanoic acid) (C9), and the like.

Furthermore, examples of the carboxylic acid having the benzene ringsubstituted with an alkyl group, an aryl group, an amino group, ahydroxyl group, an alkoxyl group, or an oxo group include, for example,toluic acid (methylbenzoic acid) (C8), xylylic acid (dimethylbenzoicacid) (C9), prehnitylic acid (2,3,4-trimethylbenzoic acid) (C10),γ-isodurylic acid (2,3,5-trimethylbenzoic acid) (C10), durylic acid(2,4,5-trimethylbenzoic acid) (C10), β-isodurylic acid(2,4,6-trimethylbenzoic acid) (C10), α-isodurylic acid(3,4,5-trimethylbenzoic acid)(C10), cumic acid (4-isopropylbenzoic acid)(C10), 4-tert-butylbenzoic acid (C11), uvitic acid (5-methylisophthalicacid) (C9), biphenyl-4-carboxylic acid (C13), diphenic acid(biphenyl-2,2′-dicarboxylic acid) (C14), dimethylaminobenzoic acid (C9),salicylic acid (2-hydroxybenzoic acid) (C7), anisic acid (methoxybenzoicacid) (C8), cresotinic acid (hydroxy (methyl) benzoic acid) (C8),o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid) (C8),m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid) (C8),p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid) (C8),o-pyrocatechuic acid (2,3-dihydroxybenzoic acid) (C7), β-resorcylic acid(2,4-dihydroxybenzoic acid) (C7), γ-resorcylic acid(2,6-dihydroxybenzoic acid) (C7), protocatechuic acid(3,4-dihydroxybenzoic acid) (C7), α-resorcylic acid(3,5-dihydroxybenzoic acid) (C7), vanillic acid(4-hydroxy-3-methoxybenzoic acid) (C8), isovanillic acid(3-hydroxy-4-methoxybenzoic acid) (C8), veratric acid(3,4-dimethoxybenzoic acid) (C9), o-veratric acid (2,3-dimethoxybenzoicacid) (C9), 2,4-dimethoxybenzoic acid (C9), orsellinic acid(2,4-dihydroxy-6-methylbenzoic acid) (C8), m-hemipinic acid(4,5-dimethoxyphthalic acid) (C10), gallic acid (3,4,5-trihydroxybenzoicacid) (C7), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid) (C9),asaronic acid (2,4,5-trimethoxybenzoic acid) (C10), mandelic acid(hydroxy (phenyl) acetic acid) (C8), vanilmandelic acid (hydroxy(4-hydroxy-3-methoxy phenyl) acetic acid) (C9), homoanisic acid((4-methoxy phenyl) acetic acid) (C9), homogentisic acid((2,5-dihydroxyphenyl) acetic acid) (C8), homoprotocatechuic acid((3,4-dihydroxyphenyl) acetic acid) (C8), homovanillic acid((4-hydroxy-3-methoxy phenyl) acetic acid) (C9), homoisovanillic acid((3-hydroxy-4-methoxy phenyl) acetic acid) (C9), homoveratric acid((3,4-dimethoxy phenyl) acetic acid) (C10), o-homoveratric acid((2,3-dimethoxy phenyl) acetic acid) (C10), homophthalic acid(2-(carboxymethyl) benzoic acid) (C9), homoisophthalic acid(3-(carboxymethyl) benzoic acid) (C9), homoterephthalic acid(4-(carboxymethyl) benzoic acid) (C9), phthalonic acid(2-(carboxycarbonyl) benzoic acid) (C9), isophthalonic acid(3-(carboxycarbonyl) benzoic acid) (C9), terephthalonic acid(4-(carboxycarbonyl) benzoic acid) (C9), atrolactic acid(2-hydroxy-2-phenylpropanoic acid) (C9), tropic acid(3-hydroxy-2-phenylpropanoic acid) (C9), melilotic acid(3-(2-hydroxyphenyl) propanoic acid) (C9), phloretic acid (3-(4-hydroxyphenyl) propanoic acid) (C9), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid) (C9), hydroferulic acid (3-(4-hydroxy-3-methoxy phenyl)propanoic acid) (C10), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl) propanoic acid) (C10), p-coumaric acid (3-(4-hydroxy phenyl)acrylic acid) (C9), umbellic acid (3-(2,4-dihydroxyphenyl) acrylic acid)(C9), caffeic acid (3-(3,4-dihydroxyphenyl) acrylic acid) (C9), ferulicacid (3-(4-hydroxy-3-methoxy phenyl) acrylic acid) (C10), isoferulicacid (3-(3-hydroxy-4-methoxy phenyl) acrylic acid) (C10), and sinapicacid (3-(4-hydroxy-3,5-dimethoxy phenyl) acrylic acid) (C11).

Examples of the carboxylic acid having the benzene ring substituted witha halogen atom include a carboxylic acid where at least one of thehydrogen atoms of benzoic acid is substituted with a fluoro group suchas fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid,tetrafluorobenzoic acid, pentafluorobenzoic acid, and the like; acarboxylic acid where at least one of the hydrogen atoms of benzoic acidis substituted with a chloro group such as chlorobenzoic acid,dichlorobenzoic acid, trichlorobenzoic acid, tetrachlorobenzoic acid,pentachlorobenzoic acid, and the like; a carboxylic acid where at leastone of the hydrogen atoms of benzoic acid is substituted with a bromogroup such as bromobenzoic acid, dibromobenzoic acid, tribromobenzoicacid, tetrabromobenzoic acid, pentabromobenzoic acid, and the like; anda carboxylic acid where at least one of the hydrogen atoms of benzoicacid is substituted with an iodine group such as iodobenzoic acid,diiodobenzoic acid, triiodobenzoic acid, tetraiodobenzoic acid,pentaiodobenzoic acid, and the like.

Specific examples of the polynuclear aromatic carboxylic acid where thecarboxyl group is directly bonded to the fused benzene ring include, forexample, 1-naphthalenecarboxylic acid, 2-naphthalenecarboxylic acid,1-anthracenecarboxylic acid, 2-anthracenecarboxylic acid,9-anthracenecarboxylic acid, phenanthrenecarboxylic acid,pyrenecarboxylic acid, and the like. Examples of the polynucleararomatic-aliphatic carboxylic acid where the aliphatic carboxylic acidis bonded to the fused benzene ring are naphthylacetic acid,naphthylpropionic acid, and the like.

The examples of the carboxylic acid having the fused benzene ringsubstituted with an alkyl group, an aryl group, an amino group, ahydroxyl group, an alkoxyl group, or an oxo group include6-amino-2-naphthalenecarboxylic acid,1,4-dihydroxy-2-naphthalenecarboxylic acid,3,5-dihydroxy-2-naphthalenecarboxylic acid,3,7-dihydroxy-2-naphthalenecarboxylic acid,3-hydroxy-2-anthracenecarboxylic acid, and9,10-dihydro-9,10-dioxo-1-anthracenecarboxylic acid.

Examples of the carboxylic acid having the fused benzene ringsubstituted with a halogen atom include fluoronaphthalenecarboxylicacid, chloronaphthalenecarboxylic acid, bromonaphthalenecarboxylic acid,fluoroanthracenecarboxylic acid, chloroanthracenecarboxylic acid, andbromoanthracenecarboxylic acid.

As (d) the salt of the carboxylic acid having the benzene ring, a saltof the above-mentioned carboxylic acid having the benzene ring isexemplified. A cation component of the salt of the carboxylic acidhaving the benzene ring may be any one of an ammonium ion, a metal ion,and an organic cation. The metal ion includes, for example, monovalentmetal ions such as sodium, potassium, lithium, silver and the like;bivalent metal ions such as magnesium, calcium, zinc, barium, cadmium,copper, cobalt, nickel, manganese and the like; trivalent metal ionssuch as aluminum, iron and the like; and other ions such as tin,zirconium, titanium and the like. Zinc ion is preferred as the cationcomponent of the carboxylic acid having the benzene ring. These cationcomponents may be used alone or as a mixture of at least two of them.

The organic cation is an organic cation having a carbon chain. Theorganic cation includes, for example, without limitation, an organicammonium ion. Examples of the organic ammonium ion are: primary ammoniumions such as stearyl ammonium ion, hexyl ammonium ion, octyl ammoniumion, 2-ethyl hexyl ammonium ion or the like; secondary ammonium ionssuch as dodecyl(lauryl) ammonium ion, octadecyl(stearyl) ammonium ion orthe like; tertiary ammonium ions such as trioctyl ammonium ion or thelike; and quaternary ammonium ions such as dioctyldimethyl ammonium ion,distearyldimethyl ammonium ion or the like. These organic cation may beused alone or as a mixture of at least two of them. The carbon number ofthe carboxylic acid having the benzene ring and/or the salt thereof isthe carbon number of the carboxylic acid component, and the carbonnumber of the organic cation is not included.

As (d) the carboxylic acid having the benzene ring and/or the saltthereof, preferred is a monocarboxylic acid having one carboxyl groupand/or a salt thereof, and more preferred are benzoic acid having onecarboxyl group directly bonded to a benzene ring and/or a derivative ofbenzoic acid having the benzoic acid structure, naphthalenecarboxylicacid having one carboxyl group directly bonded to naphthalene and/or aderivative of naphthalenecarboxylic acid having thenaphthalenecarboxylic acid structure, and anthracenecarboxylic acidhaving one carboxyl group directly bonded to anthracene and/or aderivative of anthracenecarboxylic acid having the anthracenecarboxylicacid structure.

As the benzoic acid having one carboxyl group directly bonded to thebenzene ring and/or the derivative of the benzoic acid having thebenzoic acid structure, a compound represented by the following formula(1), or a salt thereof is preferable.

(In formula (1), R¹¹ to R¹⁵ each independently represent a hydrogenatom, a halogen atom, an alkyl group having 1 to 4 carbon atoms, an arylgroup having 6 to 10 carbon atoms, an alkoxyl group having 1 to 4 carbonatoms, an amino group which may be substituted (—NR^(a)R^(b): R^(a) andR^(b) each independently are a hydrogen atom, an alkyl group having 1 to4 carbon atoms or a phenyl group) and a hydroxyl group.)

As the naphthalenecarboxylic acid having one carboxyl group directlybonded to naphthalene and/or the derivative of the naphthalenecarboxylicacid having the naphthalene carboxylic acid structure, a compoundrepresented by the following formula (2-1) or (2-2), or a salt thereofis preferred.

(In formulae (2-1) and (2-2), R²¹ to R²⁷ each independently represent ahydrogen atom, a halogen atom, an alkyl group having 1 to 4 carbonatoms, an aryl group having 6 to 10 carbon atoms, an alkoxyl grouphaving 1 to 4 carbon atoms, an amino group which may be substituted(—NR^(a)R^(b): R^(a) and R^(b) each independently are a hydrogen atom,an alkyl group having 1 to 4 carbon atoms or a phenyl group) and ahydroxyl group.)

As the anthracenecarboxylic acid having one carboxyl group directlybonded to anthracene and/or the derivative of the anthracenecarboxylicacid having the anthracenecarboxylic acid structure, a compoundrepresented by the following formula (3-1), (3-2), (3-3), or a saltthereof is preferred.

(In formulae (3-1), (3-2) and (3-3), R³¹ to R³⁹ each independentlyrepresent a hydrogen atom, a halogen atom, an alkyl group having 1 to 4carbon atoms, an aryl group having 6 to 10 carbon atoms, an alkoxylgroup having 1 to 4 carbon atoms, an amino group which may besubstituted (—NR^(a)R^(b): R^(a) and R^(b) each independently are ahydrogen atom, an alkyl group having 1 to 4 carbon atoms or a phenylgroup) and a hydroxyl group.)

(d) The carboxylic acid having the benzene ring and/or the salt thereofpreferably includes benzoic acid, buthylbenzoic acid, anisic acid(methoxybenzoic acid), dimethoxybenzoic acid, tirmethoxybenzoic acid,dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid,trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid,naphthalenecarboxylic acid, anthracenecarboxylic acid, or a saltthereof. (d) The carboxylic acid having the benzene ring and/or the saltthereof may be used alone or as a mixture of at least two of them.

The content of (d) the carboxylic acid having the benzene ring and/orthe salt thereof is preferably 2 parts by mass or more, more preferably2.5 parts by mass or more, even more preferably 3 parts by mass or more,and is preferably 40 parts by mass or less, more preferably 35 parts bymass or less, even more preferably 30 parts by mass or less with respectto 100 parts by mass of (a) the base rubber. If the content is toolittle, the effect of adding (d) the carboxylic acid having the benzenering and/or the salt thereof is not sufficient, and thus the degree ofthe outer-hard inner-soft structure of the spherical core may belowered. If the content is too much, the resilience of the core may belowered, since the hardness of the resultant core may be lowered as awhole. There are cases where the surface of zinc acrylate used as theco-crosslinking agent is treated with the carboxylic acid having thebenzene ring and/or the salt thereof to improve the dispersibility tothe rubber. In the case of using zinc acrylate whose surface is treatedwith the carboxylic acid having the benzene ring and/or the saltthereof, in the present invention, the amount of the carboxylic acidhaving the benzene ring and/or the salt thereof used as a surfacetreating agent is included in the content of (d) the carboxylic acidhaving the benzene ring and/or the salt thereof. For example, if 25parts by mass of zinc acrylate whose surface treatment amount with thecarboxylic acid having the benzene ring and/or the salt thereof is 10mass % is used, the amount of the carboxylic acid having the benzenering and/or the salt thereof is 2.5 parts by mass and the amount of zincacrylate is 22.5 parts by mass. Thus, 2.5 parts by mass is counted asthe content of (d) the carboxylic acid having the benzene ring and/orthe salt thereof.

In the case that the rubber composition used in the present inventioncontains only the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms as the co-crosslinking agent, the rubber composition furthercontains (e) a metal compound as an essential component. (e) The metalcompound is not limited, as long as it can neutralize (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubbercomposition. (e) The metal compound includes, for example, metalhydroxides such as magnesium hydroxide, zinc hydroxide, calciumhydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide,copper hydroxide, and the like; metal oxides such as magnesium oxide,calcium oxide, zinc oxide, copper oxide, and the like; metal carbonatessuch as magnesium carbonate, zinc carbonate, calcium carbonate, sodiumcarbonate, lithium carbonate, potassium carbonate, and the like. Amongthese, (e) the metal compound preferably includes a divalent metalcompound, more preferably includes a zinc compound. The divalent metalcompound reacts with the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms, thereby forming a metal crosslinking. Use of the zinccompound provides a golf ball with excellent resilience. (e) These metalcompounds are used solely or as a mixture of at least two of them.

The rubber composition used in the present invention preferably furthercontains (f) an organic sulfur compound. By using (d) the carboxylicacid having the benzene ring and/or the salt thereof and (f) the organicsulfur compound in combination for the rubber composition, the degree ofthe outer-hard and inner-soft structure of the core can be controlled.(f) The organic sulfur compound is not particularly limited, as long asit is an organic compound having a sulfur atom in the molecule thereof.Examples thereof include an organic compound having a thiol group (—SH),a polysulfide bond having 2 to 4 sulfur atoms (—S—S—, —S—S—S—, or—S—S—S—S—), or a metal salt thereof (—SM, —S-M-S—, —S-M-S—S—,—S—S-M-S—S—, —S-M-S—S—S—, or the like; M is a metal atom). Furthermore,(f) the organic sulfur compound may be any one of aliphatic compounds(aliphatic thiol, aliphatic thiocarboxylic acid, aliphaticdithiocarboxylic acid, aliphatic polysulfides, or the like),heterocyclic compounds, alicyclic compounds (alicyclic thiol, alicyclicthiocarboxylic acid, alicyclic dithiocarboxylic acid, alicyclicpolysulfides, or the like), and aromatic compounds. (f) The organicsulfur compound includes, for example, thiophenols, thionaphthols,polysulfides, thiocarboxylic acids, dithiocarboxylic acids,sulfenamides, thiurams, dithiocarbamates, and thiazoles. From the aspectof the larger hardness distribution of the spherical core, (f) theorganic sulfur compound preferably includes, organic compounds having athiol group (—SH) or a metal salt thereof, more preferably thiophenols,thionaphthols, or a metal salt thereof. Examples of the metal salts aresalts of monovalent metals such as sodium, lithium, potassium, copper(I), and silver (I), and salts of divalent metals such as zinc,magnesium, calcium, strontium, barium, titanium (II), manganese (II),iron (II), cobalt (II), nickel(II), zirconium(II), and tin (II).

Examples of the thiophenols include, for example, thiophenol;thiophenols substituted with a fluoro group, such as 2-fluorothiophenol,4-fluorothiophenol, 2,4-difluorothiophenol, 2,5-difluorothiophenol,2,6-difluorothiophenol, 2,4,5-trifluorothiophenol,2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol and the like;thiophenols substituted with a chloro group, such as 2-chlorothiophenol,4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol,2,4,5,6-tetrachiorothiophenol, pentachlorothiophenol and the like;thiophenols substituted with a bromo group, such as 2-bromothiophenol,4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol,2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like;thiophenols substituted with an iodo group, such as 2-iodothiophenol,4-iodothiophenol, 2,4-diiodothiophenol, 2,5-diiodothiophenol,2,6-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like; or ametal salt thereof. As the metal salt, zinc salt is preferred.

Examples of the naphthalenethiols (thionaphthols) are2-naphthalenethiol, 1-naphthalenethiol, 2-chloro-1-naphthalenethiol,2-bromo-1-naphthalenethiol, 2-fluoro-1-naphthalenethiol,2-cyano-1-naphthalenethiol, 2-acetyl-1-naphthalenethiol,1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol,1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and1-acetyl-2-naphthalenethiol and metal salts thereof. Preferable examplesinclude 1-naphthalenethiol, 2-naphthalenethiol and zinc salt thereof.

The sulfenamide based organic sulfur compound includes, for example,N-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. The thiuram based organic sulfurcompound includes, for example, tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.The dithiocarbamates include, for example, zinc dimethyldithiocarbamate,zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethyiphenyl dithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. The thiazole based organic sulfur compoundincludes, for example, 2-mercaptobenzothiazole (MBT), dibenzothiazyldisulfide (MBTS), sodium salt, zinc salt, copper salt, orcyclohexylamine salt of 2-mercaptobenzothiazole, 2-(2,4-dinitrophenyl)mercaptobenzothiazole, and 2-(2,6-diethyl-4-morpholinothio)benzothiazole.

(f) The organic sulfur compound can be used solely or as a mixture of atleast two of them.

The content of (f) the organic sulfur compound is preferably 0.05 partby mass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe content of (0 the organic sulfur compound is less than 0.05 part bymass, the effect of adding (f) the organic sulfur compound cannot beobtained and thus the resilience may not improve. If the content of (f)the organic sulfur compound exceeds 5.0 parts by mass, the compressiondeformation amount of the obtained golf ball becomes large and thus theresilience may be lowered.

The rubber composition used in the present invention may includeadditives such as a pigment, a filler for adjusting weight or the like,an antioxidant, a peptizing agent, and a softener where necessary.Further, as described above, if the rubber composition used in thepresent invention contains only the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms as a co-crosslinking agent, the rubbercomposition preferably contains (e) the metal compound.

Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment. As the whitepigment, titanium oxide is preferably used. The type of titanium oxideis not particularly limited, but rutile type is preferably used becauseof the high opacity. The blending amount of titanium oxide is preferably0.5 part by mass or more, and more preferably 2 parts by mass or more,and is preferably 8 parts by mass or less, and more preferably 5 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber.

It is also preferred that the rubber composition contains both a whitepigment and a blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. Examples of the purplepigment include anthraquinone violet, dioxazine violet, and methylviolet.

The blending amount of the blue pigment is preferably 0.001 part by massor more, and more preferably 0.05 part by mass or more, and ispreferably 0.2 part by mass or less, and more preferably 0.1 part bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the blending amount of the blue pigment is less than 0.001 part bymass, blueness is insufficient, and the color looks yellowish. If theblending amount of the blue pigment exceeds 0.2 part by mass, bluenessis excessively strong, and a vivid white appearance is not provided.

The filler blended in the rubber composition is mainly used as a weightadjusting agent for adjusting the weight of the golf ball obtained as afinal product. The filler may be blended where necessary. The fillerincludes, for example, inorganic fillers such as zinc oxide, bariumsulfate, calcium carbonate, magnesium oxide, tungsten powder, molybdenumpowder, or the like. The filler preferably includes zinc oxide. It isconceivable that zinc oxide functions as a vulcanization accelerator toenhance the hardness of the spherical core as a whole. The content ofthe filler is preferably 0.5 part by mass or more, more preferably 1part by mass or more, and is preferably 30 parts by mass or less, morepreferably 25 parts by mass or less, even more preferably 20 parts bymass or less. If the content of the filler is less than 0.5 part bymass, it is difficult to adjust the weight, while if the content of thefiller exceeds 30 parts by mass, the weight ratio of the rubbercomponent is reduced and thus the resilience tends to be lowered.

The blending amount of the antioxidant is preferably 0.1 part by mass ormore and 1 part by mass or less, with respect to 100 parts by mass of(a) the base rubber. In addition, the blending amount of the peptizingagent is preferably 0.1 part by mass or more and 5 parts by mass orless, with respect to 100 parts by mass of (a) the base rubber.

The rubber composition used in the present invention is obtained bymixing and kneading (a) the base rubber, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal saltthereof, (c) the crosslinking initiator, and (d) the carboxylic acidhaving the benzene ring or the salt thereof, and other additives wherenecessary. The kneading can be conducted, without any limitation, with awell-known kneading machine such as a kneading roll, a banbury mixer, akneader, or the like.

The spherical core of the golf ball of the present invention can beobtained by molding the rubber composition after kneaded. Thetemperature for molding the spherical core is preferably 120° C. ormore, more preferably 150° C. or more, even more preferably 160° C. ormore, and is preferably 170° C. or less. If the molding temperatureexceeds 170° C., the surface hardness of the core tends to decrease. Themolding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The moldingtime preferably ranges from 10 minutes to 60 minutes.

In a preferable embodiment, when the hardness is measured at nine pointsobtained by dividing a radius of the spherical core into equal partshaving 12.5% interval and the hardness is plotted against distance (%)from the center of the spherical core, the spherical core is such thatR² of a linear approximation curve obtained by the least square methodis 0.90 or higher, more preferably 0.95 or higher. If R² is 0.90 ormore, the linearity of the core hardness distribution is enhanced, thusthe spin rate on driver shots decreases, resulting in the greater flightdistance.

The hardness of the spherical core is JIS-C hardness measured at ninepoints obtained by dividing a radius of the spherical core into equalparts having 12.5% interval. That is, JIS-C hardness is measured at ninepoints, namely at distances of 0% (core center), 12.5%, 25%, 37.5%, 50%,62.5%, 75%, 87.5%, 100% (core surface) from the core center. Next, themeasurement results are plotted to make a graph having JIS-C hardness asa vertical axis and distances (%) from the core center as a horizontalaxis. In the present invention, R² of a linear approximation curveobtained from this graph by the least square method is preferably 0.90or higher, more preferably 0.95 or higher. R² of the linearapproximation curve obtained by the least square method indicates thelinearity of the obtained plot. In the present invention, R² of 0.90 ormore means that the core has the hardness distribution where thehardness increases linearly or almost linearly. If the core having thehardness distribution where the hardness increases linearly or almostlinearly is used for the golf ball, the spin rate on driver shotsdecrease. As a result, the flight distance on driver shots increases. R²of the linear approximation curve is preferably 0.96 or more. The higherlinearity provides a greater flight distance on driver shots.

The spherical core preferably has a hardness difference (Hs−Ho) betweena surface hardness Hs and a center hardness Ho of 30 or more, morepreferably 33 or more, even more preferably 35 or more, and preferablyhas a hardness difference of 80 or less, more preferably 70 or less,even more preferably 60 or less in JIS-C hardness. If the hardnessdifference between the center hardness and the surface hardness islarge, the golf ball having a great flight distance due to the highlaunch angle and low spin rate is obtained.

The spherical core preferably has the center hardness Ho of 30 or more,more preferably 40 or more, even more preferably 45 or more in JIS-Chardness. If the center hardness Ho is less than 30 in JIS-C hardness,the core becomes too soft and thus the resilience may be lowered.Further, the spherical core preferably has the center hardness Ho of 70or less, more preferably 65 or less, even more preferably 60 or less inJIS-C hardness. If the center hardness Ho exceeds 70 in JIS-C hardness,the core becomes too hard and thus the shot feeling tends to be lowered.

The spherical core preferably has the surface hardness Hs of 76 or more,more preferably 78 or more, and preferably has the surface hardness Hsof 100 or less, more preferably 95 or less in JIS-C hardness. If thesurface hardness of the spherical core is 76 or more in JIS-C hardness,the spherical core does not become excessively soft, and thus the betterresilience is obtained. Further, if the surface hardness of thespherical core is 100 or less in JIS-C hardness, the spherical core doesnot become excessively hard, and thus the better shot feeling isobtained.

The spherical core preferably has the diameter of 34.8 mm or more, morepreferably 36.8 mm or more, and even more preferably 38.8 mm or more,and preferably has the diameter of 42.2 mm or less, more preferably 41.8mm or less, and even more preferably 41.2 mm or less, and mostpreferably 40.8 mm or less. If the spherical core has the diameter of34.8 mm or more, the thickness of the cover does not become too thickand thus the resilience becomes better. On the other hand, if thespherical core has the diameter of 42.2 mm or less, the thickness of thecover does not become too thin, and thus the cover functions better.

When the spherical core has a diameter from 34.8 mm to 42.2 mm, acompression deformation amount (shrinking deformation amount of thespherical core along the compression direction) of the spherical corewhen applying a load from 98 N as an initial load to 1275 N as a finalload is preferably 2.0 mm or more, more preferably 2.8 mm or more, andis preferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling ofthe golf ball becomes better. If the compression deformation amount is6.0 mm or less, the resilience of the golf ball becomes better.

The golf ball cover of the present invention is formed from a covercomposition containing a resin component. Examples of the resincomponent include, for example, an ionomer rein; a thermoplasticpolyurethane elastomer having a commercial name of “Elastollan”commercially available from BASF Japan Ltd; a thermoplastic polyamideelastomer having a commercial name of “Pebax” commercially availablefrom Arkema K. K.; a thermoplastic polyester elastomer having acommercial name of “Hytrel” commercially available from Du Pont-TorayCo., Ltd.; and a thermoplastic styrene elastomer having a commercialname of “Rabalon” commercially available from Mitsubishi ChemicalCorporation; and the like.

The ionomer resin includes a product prepared by neutralizing at least apart of carboxyl groups in the binary copolymer composed of an olefinand an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with ametal ion, a product prepared by neutralizing at least a part ofcarboxyl groups in the ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and anα,β-unsaturated carboxylic acid ester with a metal ion, or a mixture ofthose. The olefin preferably includes an olefin having 2 to 8 carbonatoms. Examples of the olefin are ethylene, propylene, butene, pentene,hexene, heptene, and octene. The olefin more preferably includesethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleicacid and crotonic acid. Among these, acrylic acid and methacrylic acidare particularly preferred. Examples of the α,β-unsaturated carboxylicacid ester include methyl ester, ethyl ester, propyl ester, n-butylester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid,maleic acid or the like. In particular, acrylic acid ester andmethacrylic acid ester are preferable. Among these, the ionomer resinpreferably includes the metal ion-neutralized product of the binarycopolymer composed of ethylene-(meth)acrylic acid and the metalion-neutralized product of the ternary copolymer composed of ethylene,(meth)acrylic acid, and (meth)acrylic acid ester.

Specific examples of the ionomer resins include trade name “Himilan(registered trademark) (e.g. the binary copolymerized ionomer such asHimilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706(Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternarycopolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))”commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples include “Surlyn (registered trademark) (e.g. thebinary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn),Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn),Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li),Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), HPF1000 (Mg), HPF 2000 (Mg))” commercially available from E.I. du Pont deNemours and Company.

Further, examples include “Iotek (registered trademark) (e.g. the binarycopolymerized ionomer such as Iotek 8000 (Na), Iotek 8030 (Na), Iotek7010 (Zn), Iotek 7030 (Zn); and the ternary copolymerized ionomer suchas Iotek 7510 (Zn), Iotek 7520 (Zn))” commercially available fromExxonMobil Chemical Corporation.

It is noted that Na, Zn, Li, and Mg described in the parentheses afterthe trade names indicate metal types of neutralizing metal ions for theionomer resins.

The cover composition constituting the cover of the golf ball of thepresent invention preferably includes, as a resin component, athermoplastic polyurethane elastomer or an ionomer rein. In case ofusing the ionomer rein, it is also preferred to use a thermoplasticstyrene elastomer together. The content of the polyurethane or ionomerresin in resin component of the cover composition is preferably 50 mass% or more, more preferably 60 mass % or more, and even more preferably70 mass % or more.

In the present invention, the cover composition may further contain apigment component such as a white pigment (for example, titanium oxide),a blue pigment, and a red pigment; a weight adjusting agent such as zincoxide, calcium carbonate, and barium sulfate; a dispersant; anantioxidant; an ultraviolet absorber; a light stabilizer; a fluorescentmaterial or a fluorescent brightener; and the like, as long as they donot impair the effect of the present invention.

The amount of the white pigment (for example, titanium oxide) ispreferably 0.5 part or more, more preferably 1 part or more, and thecontent of the white pigment is preferably 10 parts or less, morepreferably 8 parts or less, with respect to 100 parts of the resincomponent constituting the cover by mass. If the amount of the whitepigment is 0.5 part by mass or more, it is possible to impart theopacity to the resultant cover. Further, if the amount of the whitepigment is more than 10 parts by mass, the durability of the resultantcover may deteriorate.

The slab hardness of the cover composition is preferably set inaccordance with the desired performance of the golf balls. For example,in case of a so-called distance golf ball which focuses on a flightdistance, the cover composition preferably has a slab hardness of 50 ormore, more preferably 55 or more, and preferably has a slab hardness of80 or less, more preferably 70 or less in shore D hardness. If the covercomposition has a slab hardness of 50 or more, the obtained golf ballhas a high launch angle and low spin rate on driver shots and ironshots, and thus the flight distance becomes large. If the covercomposition has a slab hardness of 80 or less, the golf ball excellentin durability is obtained. Further, in case of a so-called spin golfball which focuses on controllability, the cover composition preferablyhas a slab hardness of less than 50, and preferably has a slab hardnessof 20 or more, more preferably 25 or more in shore D hardness. If thecover composition has a slab hardness of less than 50, the flightdistance on driver shots can be improved by the core of the presentinvention, as well as the obtained golf ball readily stops on the greendue to the high spin rate on approach shots. If the cover compositionhas a slab hardness of 20 or more, the abrasion resistance improves. Incase of a plurality of cover layers, the slab hardness of the covercomposition constituting each layer can be identical or different, aslong as the slab hardness of each layer is within the above range.

An embodiment for molding a cover is not particularly limited, andincludes an embodiment which comprises injection molding the covercomposition directly onto the core, or an embodiment which comprisesmolding the cover composition into a hollow-shell, covering the corewith a plurality of the hollow-shells and subjecting the core with aplurality of the hollow shells to the compression-molding (preferably anembodiment which comprises molding the cover composition into a halfhollow-shell, covering the core with the two half hollow-shells, andsubjecting the core with the two half hollow-shells to thecompression-molding).

When molding the cover in a compression molding method, molding of thehalf shell can be performed by either compression molding method orinjection molding method, and the compression molding method ispreferred. The compression-molding of the cover composition into halfshell can be carried out, for example, under a pressure of 1 MPa or moreand 20 MPa or less at a temperature of −20° C. or more and 70° C. orless relative to the flow beginning temperature of the covercomposition. By performing the molding under the above conditions, ahalf shell having a uniform thickness can be formed. Examples of amethod for molding the cover using half shells include compressionmolding by covering the core with two half shells. The compressionmolding of half shells into the cover can be carried out, for example,under a pressure of 0.5 MPa or more and 25 MPa or less at a temperatureof −20° C. or more and 70° C. or less relative to the flow beginningtemperature of the cover composition. By performing the molding underthe above conditions, a golf ball cover having a uniform thickness canbe formed.

In the case of directly injection molding the cover composition, thecover composition extruded in the pellet form beforehand may be used forinjection molding or the materials such as the base resin components andthe pigment may be dry blended, followed by directly injection moldingthe blended material. It is preferred to use upper and lower moldshaving a spherical cavity and pimples for forming a cover, wherein apart of the pimples also serves as a retractable hold pin. When moldingthe cover by injection molding, the hold pin is protruded to hold thecore, and the cover composition which has been heated and melted ischarged and then cooled to obtain a cover. For example, it is preferredthat the cover composition heated and melted at the temperature rangingfrom 200° C. to 250° C. is charged into a mold held under the pressureof 9 MPa to 15 MPa for 0.5 to 5 seconds, and after cooling for 10 to 60seconds, the mold is opened and the golf ball with the cover molded istaken out from the mold.

The concave portions called “dimple” are usually formed on the surfaceof the cover. The total number of the dimples is preferably 200 or moreand 500 or less. If the total number is less than 200, the dimple effectis hardly obtained. On the other hand, if the total number exceeds 500,the dimple effect is hardly obtained because the size of the respectivedimples is small. The shape (shape in a plan view) of dimples includes,for example, without limitation, a circle, polygonal shapes such asroughly triangular shape, roughly quadrangular shape, roughly pentagonalshape, roughly hexagonal shape, and another irregular shape. The shapeof the dimples is employed solely or at least two of them may be used incombination.

In the present invention, the thickness of the cover of the golf ball ispreferably 4.0 mm or less, more preferably 3.0 mm or less, even morepreferably 2.0 mm or less. If the thickness of the cover is 4.0 mm orless, the resilience and shot feeling of the obtained golf ball becomebetter. The thickness of the cover is preferably 0.3 mm or more, morepreferably 0.5 mm or more, and even more preferably 0.8 mm or more, andmost preferably 1.0 mm or more. If the thickness of the cover is lessthan 0.3 mm, the durability and the wear resistance of the cover maydeteriorate. If the cover has a plurality of layers, it is preferredthat the total thickness of the cover layers falls within the aboverange.

After the cover is molded, the mold is opened and the golf ball body istaken out from the mold, and as necessary, the golf ball body ispreferably subjected to surface treatments such as deburring, cleaning,and sandblast. If desired, a paint film or a mark may be formed. Thepaint film preferably has a thickness of, but not limited to, 5 μm orlarger, and more preferably 7 μm or larger, and preferably has athickness of 50 μm or smaller, and more preferably 40 μm or smaller,even more preferably 30 μm or smaller. If the thickness is smaller than5 μm, the paint film is easy to wear off due to continued use of thegolf ball, and if the thickness is larger than 50 μm, the effect of thedimples is reduced, resulting in lowering flying performance of the golfball.

When the golf ball of the present invention has a diameter in a rangefrom 40 mm to 45 mm, a compression deformation amount of the golf ball(shrinking amount of the golf ball in the compression direction thereof)when applying a load from an initial load of 98 N to a final load of1275 N to the golf ball is preferably 2.0 mm or more, more preferably2.4 mm or more, even more preferably 2.5 mm or more, most preferably 2.8mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm orless. If the compression deformation amount is 2.0 mm or more, the golfball does not become excessively hard, and thus exhibits the good shotfeeling. On the other hand, if the compression deformation amount is 5.0mm or less, the resilience is enhanced.

The golf ball construction is not limited, as long as the golf ball ofthe present invention comprises a spherical core and at least one coverlayer covering the spherical core. FIG. 1 is a partially cutawaysectional view showing the golf ball 2 according to the preferableembodiment of the present invention. The golf ball 2 comprises aspherical core 4, and a cover 12 covering the spherical core 4.Plurality of dimples 14 are formed on a surface of the cover. Otherportions than dimples 14 on the surface of the golf ball 2 are land 16.The golf ball 2 is provided with a paint layer and a mark layer outsidethe cover 12, but these layers are not depicted.

The spherical core preferably has a single layered structure. Unlike themulti-layered structure, the spherical core of the single layeredstructure does not have an energy loss at the interface of themulti-layered structure when hitting, and thus has an improvedresilience. The cover has a structure of at least one layer, for examplea single layered structure, or a multi-layered structure of at least twolayers. The golf ball of the present invention includes, for example, atwo-piece golf ball comprising a spherical core and a single layeredcover disposed around the spherical core, a multi-piece golf ballcomprising a spherical core, and at least two cover layers disposedaround the spherical core (including the three-piece golf ball), and awound golf ball comprising a spherical core, a rubber thread layer whichis formed around the spherical core, and a cover disposed over therubber thread layer. The present invention can be suitably applied toany one of the above golf balls.

EXAMPLES

Hereinafter, the present invention will be described in detail by way ofexample. The present invention is not limited to examples describedbelow. Various changes and modifications can be made without departingfrom the spirit and scope of the present invention.

[Evaluation Methods] (1) Compression Deformation Amount (mm)

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball in the compression direction thereof),when applying a load from 98 N as an initial load to 1275 N as a finalload to the core or golf ball, was measured.

(2) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition, and stored at 23° C. for two weeks. Three or moreof these sheets were stacked on one another so as not to be affected bythe measuring substrate on which the sheets were placed, and thehardness of the stack was measured with a type P1 auto loading durometermanufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D typespring hardness tester prescribed in ASTM-D2240.

(3) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co.,Ltd., provided with a JIS—C type spring hardness tester was used tomeasure the hardness of the spherical core. The hardness measured at thesurface of the spherical core was adopted as the surface hardness of thespherical core. The spherical core was cut into two hemispheres toobtain a cut plane, and the hardness were measured at the central pointand at predetermined distances from the central point. The core hardnesswere measured at 4 points of the predetermined distances from thecentral point of the cut plane of the core. The core hardness wascalculated by averaging the hardness measured at 4 points of thepredetermined distances.

(4) Spin Rate (rpm) on a Driver Shot

A metal-headed W#1 driver (XXIO S, loft 11°, manufactured by SRI SportsLimited) was installed on a swing robot M/C manufactured by GolfLaboratories, Inc. A golf ball was hit at a head speed of 40 m/sec, andthe spin rates right after hitting the golf ball were measured. Thismeasurement was conducted twelve times for each golf ball, and theaverage value was adopted as the measurement value for the golf ball. Asequence of photographs of the hit golf ball were taken for measuringthe spin rate (rpm) right after hitting the golf ball. In tables 3 to 8,the spin rate on the driver shots of golf balls are shown as thedifference from those of the golf ball (core) No. 20.

[Production of Golf Balls] (1) Production of Cores

The rubber compositions having formulations shown in Tables 3 to 8 werekneaded and heat-pressed in upper and lower molds, each having ahemispherical cavity, at 170° C. for 20 minutes to prepare sphericalcores having a diameter of 39.8 mm.

TABLE 3 Golf ball No. 1 2 3 4 5 Rubber BR730 100 100 100 100 100composition Sanceler SR 29 31 32 37 35 (parts by mass) Zinc oxide 5 5 55 5 Barium sulfate *1) *1) *1) *1) *1) 2-Thionaphthol 0.1 0.1 0.1 0.10.1 Dimethylaminobenzoic acid 3 5 10 20 30 Anisic acid — — — — — Benzoicacid — — — — — Chlorobenzoic acid — — — — — Dicumyl peroxide 0.8 0.8 0.80.8 0.8 Core hardness Center hardness 46.5 45.6 47.2 49.0 56.0distribution 12.5% point hardness 53.9 50.7 51.4 53.4 59.2 (JIS-C) 25%point hardness 61.8 58.5 57.5 59.2 62.9 37.5% point hardness 65.4 63.360.6 62.5 65.5 50% point hardness 66.2 64.4 65.5 66.0 68.5 62.5% pointhardness 68.6 67.4 73.2 71.1 72.7 75% point hardness 78.5 78.0 78.7 77.976.3 87.5% point hardness 79.5 79.6 76.0 78.5 75.3 Surface hardness 84.283.5 82.3 84.8 82.5 Surface hardness − center hardness 37.7 37.9 35.135.8 26.5 R² of approximated curve 0.96 0.98 0.97 0.99 0.98 Slope ofapproximated curve 0.35 0.38 0.36 0.35 0.25 Core compression deformationamount (mm) 4.07 3.93 3.96 3.97 4.03 Cover composition A A A A A Coverhardness (Shore D) 65 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.51.5 Ball Driver spin rate (rpm) −80 −130 −90 −80 −30 Compressiondeformation amount (mm) 3.37 3.23 3.26 3.27 3.33

TABLE 4 Golf ball No. 6 7 8 9 10 11 Rubber BR730 100 100 100 100 100 100composition Sanceler SR 31 37 27 31 24 28 (parts by mass) Zinc oxide 5 55 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) 2-Thionaphthol 0.1 0.10.1 0.1 0.1 0.1 Dimethylaminobenzoic acid — — — — — — Anisic acid 2.8 5— — — — Benzoic acid — — 2.3 5 10 20 Chlorobenzoic acid — — — — — —Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 Core hardness Center hardness45.7 49.2 47.7 45.9 45.9 49.5 distribution 12.5% point hardness 54.953.6 59.2 51.8 53.3 55.0 (JIS-C) 25% point hardness 61.9 59.0 66.4 59.061.2 61.8 37.5% point hardness 65.2 61.6 69.1 64.2 65.2 64.5 50% pointhardness 65.7 63.2 69.4 66.5 66.8 67.6 62.5% point hardness 66.2 68.968.5 65.4 65.9 72.4 75% point hardness 74.9 75.2 76.0 77.7 77.4 75.387.5% point hardness 77.3 77.4 80.2 83.0 80.7 71.9 Surface hardness 83.283.3 86.4 89.3 86.9 75.9 Surface hardness − center hardness 37.5 34.138.7 43.4 41.0 26.4 R² of approximated curve 0.94 0.99 0.90 0.96 0.950.90 Slope of approximated curve 0.33 0.33 0.32 0.41 0.37 0.25 Corecompression deformation amount (mm) 3.97 4.02 4.02 4.08 3.96 3.99 Covercomposition A A A A A A Cover hardness (Shore D) 65 65 65 65 65 65 Coverthickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −70−90 −60 −120 −100 −20 Compression deformation amount (mm) 3.27 3.32 3.323.38 3.26 3.29

TABLE 5 Golf ball No. 12 13 14 15 16 Rubber BR730 100 100 100 100 100composition Sanceler SR 31 40 29 34 39 (parts by mass) Zinc oxide 5 5 55 5 Barium sulfate *1) *1) *1) *1) *1) 2-Thionaphthol 0.1 0.1 0.1 0.10.1 Dimethylaminobenzoic acid — — — — — Acetoxybenzoic acid — — 2.5 5.17.6 Benzoic acid — — — — — Chlorobenzoic acid 5 10 — — — Dicumylperoxide 0.8 0.8 0.8 0.8 0.8 Core hardness Center hardness 47.3 52.348.5 45.1 42.0 distribution 12.5% point hardness 55.1 55.9 58.4 51.046.5 (JIS-C) 25% point hardness 62.0 61.1 65.3 57.7 51.9 37.5% pointhardness 65.1 64.0 67.8 61.4 54.9 50% point hardness 65.8 67.6 69.1 63.656.0 62.5% point hardness 69.7 73.2 68.1 62.7 57.2 75% point hardness76.1 77.4 71.8 70.0 66.5 87.5% point hardness 75.2 75.9 78.1 75.4 71.5Surface hardness 81.7 83.7 85.0 82.9 79.4 Surface hardness − centerhardness 34.4 31.4 36.5 37.8 37.4 R² of approximated curve 0.95 0.980.90 0.96 0.95 Slope of approximated curve 0.31 0.30 0.29 0.33 0.34 Corecompression deformation amount (mm) 3.96 4.03 3.99 3.99 4.04 Covercomposition A A A A A Cover hardness (Shore D) 65 65 65 65 65 Coverthickness (mm) 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −80 −85−40 −80 −70 Compression deformation amount (mm) 3.26 3.33 3.29 3.29 3.34

TABLE 6 Golf ball No. 17 18 19 20 21 Rubber BR730 100 100 100 100 100composition Sanceler SR 27 26 25 23 27 (parts by mass) Zinc oxide 5 5 55 5 Barium sulfate *1) *1) *1) *1) *1) 2-Thionaphthol 0.1 0.1 0.1 — 0.1Dimethylaminobenzoic acid 1 — — — — Anisic acid — 0.93 — — — Benzoicacid — — 0.74 — — Chlorobenzoic acid — — — — — Dicumyl peroxide 0.8 0.80.8 0.8 0.8 Core hardness Center hardness 53.2 51.7 54.0 56.3 58.2distribution 12.5% point hardness 63.6 62.9 64.5 61.1 63.7 (JIS-C) 25%point hardness 69.4 68.6 69.3 65.2 68.1 37.5% point hardness 71.3 70.270.7 66.9 69.7 50% point hardness 71.2 70.4 71.1 66.9 70.0 62.5% pointhardness 69.3 69.1 70.0 67.2 68.8 75% point hardness 74.5 73.8 73.6 71.573.9 87.5% point hardness 79.0 78.4 78.3 71.9 77.3 Surface hardness 85.585.0 85.0 79.2 83.0 Surface hardness − center hardness 82.3 33.3 31.022.9 24.8 R² of approximated curve 0.85 0.85 0.85 0.91 0.90 Slope ofapproximated curve 0.24 0.25 0.23 0.18 0.20 Core compression deformationamount (mm) 4.01 4.02 4.06 4.35 4.01 Cover composition A A A A A Coverhardness (Shore D) 65 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.51.5 Ball Driver spin rate (rpm) 10 0 15 0 −10 Compression deformationamount (mm) 3.31 3.32 3.36 3.65 3.31

TABLE 7 Golf ball No. 22 23 24 25 26 27 Rubber BR730 100 100 100 100 100100 composition Sanceler SR 31 31 29 33 29 31 (parts by mass) Zinc oxide5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) 2-Thionaphthol 0.10.1 0.1 0.1 0.1 0.1 2,4-dimethoxybenzoic acid 2.6 — — — — —2,4,5-trimethoxybenzoic acid — 3 — — — — 2,4-dichlorobenzoic acid — —5.4 — — — 2,4,6-trichlorobenzoic acid — — — 6.4 — — 4-tert-butylbenzoicacid — — — — 5 — Biphenyl-4-carboxylic acid — — — — — 5.6 Dicumylperoxide 0.8 0.8 0.8 0.8 0.8 0.8 Core hardness Center hardness 44.1 44.746.0 48.6 49.0 44.7 distribution 12.5% point hardness 53.9 53.3 54.859.1 57.9 53.9 (JIS-C) 25% point hardness 61.0 60.7 61.9 68.0 63.2 60.437.5% point hardness 64.2 63.5 65.2 70.9 66.4 63.2 50% point hardness64.9 63.8 65.5 71.3 66.7 63.5 62.5% point hardness 65.6 65.5 69.4 70.068.4 66.8 75% point hardness 75.1 73.4 76.3 75.9 76.3 74.7 87.5% pointhardness 77.1 75.5 77.3 77.9 78.1 76.6 Surface hardness 83.9 83.5 83.385.3 86.1 82.6 Surface hardness − center hardness 39.8 38.9 37.3 36.837.2 37.9 R² of approximated curve 0.94 0.95 0.96 0.87 0.95 0.96 Slopeof approximated curve 0.34 0.33 0.33 0.29 0.32 0.34 Core compressiondeformation amount (mm) 4.0 4.0 4.2 3.8 3.9 4.1 Cover composition A A AA A A Cover hardness (Shore D) 65 65 65 65 65 65 Cover thickness (mm)1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −90 −90 −75 −50 −70−80 Compression deformation amount (mm) 3.30 3.30 3.50 3.10 3.20 3.40

TABLE 8 Golf ball No. 28 29 30 Rubber BR730 100 100 100 compositionSanceler SR 33 33 31 (parts by Zinc oxide 5 5 5 mass) Barium sulfate *1)*1) *1) 2-Thionaphthol 0.1 0.1 0.1 1-naphthalenecarboxyic acid 4.8 — —2-naphthalenecarboxylic acid — 4.8 — 9-anthracenecarboxylic acid — — 6.2Dicumyl peroxide 0.8 0.8 0.8 Core Center hardness 49.8 49.9 49.6hardness 12.5% point hardness 55.3 56.1 55.9 distribution 25% pointhardness 61.3 61.6 61.9 (JIS-C) 37.5% point hardness 63.6 63.9 64.2 50%point hardness 65.0 65.1 64.7 62.5% point hardness 71.4 70.7 65.6 75%point hardness 77.2 76.7 72.9 87.5% point hardness 77.5 78.0 75.4Surface hardness 83.9 84.6 81.1 Surface hardness − 34.2 34.7 31.5 centerhardness R² of approximated curve 0.98 0.98 0.95 Slope of approximatedcurve 0.32 0.32 0.28 Core compression deformation amount (mm) 4.1 4.04.1 Cover composition A A A Cover hardness (Shore D) 65 65 65 Coverthickness (mm) 1.5 1.5 1.5 Ball Driver spin rate (rpm) −90 −90 −70Compression deformation 3.30 3.30 3.50 amount (mm) *1) In tables No. 3to 8, as to an amount of barium sulfate, adjustment was made such thatthe golf ball had a mass of 45.4 g.BR730: a high-cis polybutadiene (cis-1,4 bond content=96 mass %,1,2-vinyl bond content=1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.)=55,molecular weight distribution (Mw/Mn)=3) available from JSR CorporationSanceler SR: zinc acrylate (product of 10 mass % stearic acid coating)available from Sanshin Chemical Industry Co., Ltd.Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd.Barium sulfate: “Barium sulfate BD” manufactured by Sakai ChemicalIndustry Co., Ltd., adjustment was made such that the finally obtainedgolf ball had a mass of 45.4 g.2-thionaphthol: available from Tokyo Chemical Industry Co., Ltd.Dicumyl peroxide: “PERCUMYL® D” available from NOF Corporation.Benzoic acid: available from Tokyo Chemical Industry Co., Ltd. (purity:98% or more)Dimethylaminobenzoic acid: 4-dimethylaminobenzoic acid available fromTokyo Chemical Industry Co., Ltd. (purity: 98% or more)Anisic acid: p-anisic acid available from Tokyo Chemical Industry Co.,Ltd. (4-methoxybenzoic acid, purity: 99% or more)Chlorobenzoic acid: available from Tokyo Chemical Industry Co., Ltd.4-chlorobenzoic acid (purity: 99% or more)Acetoxybenzoic acid: 4-acetoxybenzoic acid available from Tokyo ChemicalIndustry Co., Ltd. (purity: 98% or more)2,4-dimethoxybenzoic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 99.0% or more)2,4,5-trimethoxybenzoic acid: available from Tokyo Chemical IndustryCo., Ltd. (purity: 98.0% or more)2,4-dichlorobenzoic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 95.0% or more)2,4,6-trichlorobenzoic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 98.0% or more)4-tert-bytylbenzoic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 99.0% or more)Biphenyl-4-carboxylic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 98.0% or more)1-naphthalenecarboxyic acid: 1-naphthoic acid available from TokyoChemical Industry Co., Ltd. (purity: 98.0% or more)2-naphthalenecarboxyic acid: 2-naphthoic acid available from TokyoChemical Industry Co., Ltd. (purity: 98.0% or more)9-anthracenecarboxylic acid: available from Tokyo Chemical Industry Co.,Ltd. (purity: 97.0% or more)

(2) Production of Cover

Cover materials shown in Table 9 were mixed with a twin-screw kneadingextruder to prepare the cover compositions in the pellet form. Theextruding conditions of the cover composition were a screw diameter of45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and themixtures were heated to 150 to 230° C. at the die position of theextruder. The cover compositions obtained above were injection-moldedonto the spherical cores to produce the golf balls having the sphericalcore and the cover covering the spherical core.

TABLE 9 Cover composition A Himilan 1605 50 Himilan 1706 50 Titaniumoxide 4 Slab hardness (Shore D) 65Formulation: parts by massHimilan 1605: Sodium ion neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., LtdHimilan 1706: Zinc ion neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd

As apparent from tables 3 to 8, the golf balls comprising a sphericalcore and at least one cover layer covering the spherical core, whereinthe spherical core is formed from a rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or a metal salt thereof as a co-crosslinking agent, (c) acrosslinking initiator and (d) a carboxylic acid having a benzene ringand/or a salt thereof have a low spin rate on driver shots,respectively, are expected to travel a great flight distance.

The golf ball of the present invention has a low spin rate on drivershots. This application is based on Japanese Patent application Nos.2011-289985 and filed on Dec. 28, 2011 and 2012-166101 filed on Jul. 26,2012.

1. A golf ball having a spherical core and at least one cover layercovering the spherical core, wherein the spherical core is formed from arubber composition containing (a) a base rubber, (b) an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofas a co-crosslinking agent, (c) a crosslinking initiator, and (d) acarboxylic acid having a benzene ring substituted with a halogen atomand/or a salt thereof, provided that the rubber composition furthercontains (e) a metal compound in the case of containing only (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent.
 2. The golf ball according to claim 1, whereinthe rubber composition contains (d) the carboxylic acid having thebenzene ring substituted with a halogen atom and/or the salt thereof ina content ranging from 2 parts by mass to 40 parts by mass with respectto 100 parts by mass of (a) the base rubber.
 3. The golf ball accordingto claim 1, wherein the (d) the carboxylic acid having the benzene ringsubstituted with a halogen atom and/or the salt thereof is a derivativeof the benzoic acid having a benzoic acid structure.
 4. The golf ballaccording to claim 3, wherein the derivative of the benzoic acid havingthe benzoic acid structure is a compound represented by a followingformula (1) and/or a salt thereof.

(In formula (1), R¹¹ to R¹⁵ each independently represent a hydrogen atomor a halogen atom, provided that at least one of R¹¹ to R¹⁵ is a halogenatom.)
 5. The golf ball according to claim 1, wherein (d) the carboxylicacid having the benzene ring substituted with a halogen atom and/or thesalt thereof includes at least one compound selected from the groupconsisting of chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoicacid, and/or a salt thereof.
 6. The golf ball according to claim 1,wherein the rubber composition further contains (f) an organic sulfurcompound.
 7. The golf ball according to claim 6, wherein (f) the organicsulfur compound includes at least one compound selected from the groupconsisting of thiophenols, diphenyl disulfides, thionaphthols, thiuramdisulfides, and metal salts thereof.
 8. The golf ball according to claim6, wherein the rubber composition contains (f) the organic sulfurcompound in a content ranging from 0.05 part to 5 parts by mass withrespect to 100 parts by mass of (a) the base rubber.
 9. The golf ballaccording to claim 1, the rubber composition contains (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof in a content ranging from 15 parts to 50 parts bymass with respect to 100 parts by mass of (a) the base rubber.
 10. Thegolf ball according to claim 1, wherein the rubber composition contains(b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent.
 11. The golf ball accordingto claim 1, wherein the spherical core is such that R² of a linearapproximation curve obtained from a least square method is 0.90 orhigher, when JIS-C hardness, which is measured at nine points obtainedby dividing a radius of the spherical core into equal parts having 12.5%intervals therebetween, is plotted against distance (%) from a corecenter.
 12. The golf ball according to claim 1, wherein the sphericalcore has a hardness difference in a range from 30 to 80 in JIS-Chardness between a surface hardness and a center hardness thereof. 13.The golf ball according to claim 1, wherein the spherical core has adiameter in a range from 34.8 mm to 42.2 mm and a compressiondeformation amount in a range from 2.0 mm to 6.0 mm when applying a loadfrom 98N as an initial load to 1275N as a final load to the sphericalcore.